The present invention relates to loudspeaker systems and, more particularly, to a new and improved loudspeaker configuration having a predetermined constant forward directional characteristic over a wide audio frequency range.
In sound reproduction applications such as auditorium speaker systems, public address systems, and home audio systems; it is desirable to provide sound radiation patterns which have front directional characteristics and have little or no acoustical radiation to the sides and rear. This is because sideward and rearward acoustical radiations cause reflection and reverberation effects which interfere with the intelligibility of sound. Additionally, such sideward and rearward radiations often result in acoustical feedback to a microphone which causes an oscillatory condition with audible howling or squealing noises.
Within an enclosed listening area sound reproduction consists of both direct and reflected acoustical radiations. Direct radiation is that portion of the sound waves which is reproduced by a speaker system and travels in a straight line to the listener. The direct sound radiation contains all of the original information as it was created. In the case of recordings this information provides the depth, clarity and the location of instruments and vocalists, as well as the reverberation effects of the original recording environment. Interference with this direct sound radiation distorts and impedes accurate sound reproduction for the listener.
Reflected acoustical radiation is caused by sound waves striking against obstacles such as floors, ceilings, walls, and furnishings. The sound waves then bounce back to both the listener and the direct sound waves radiated from the speaker system. The reflected sound creates interference with the direct acoustical radiation bv adding to or subtracting from the intensity of the direct sound waves and by causing reverberation effects. Reverberation, better known as echo, results from multiple reflections at successively decreasing intensities which persist after the original sound has stopped. These interference effects are more pronounced when the listening room is small or obstacles are close to the speaker system. In stereo reproduction these interference effects cause a scattering of the sound image which should exist between the two speaker systems. Superimposing the acoustical properties and reverberations of the listening area on the original sound is undesirable for accurate sound reproduction. Thus, by eliminating sideward and rearward acoustical radiation patterns and creating frontal dispersion characteristics, reflected sound can be reduced to negligible levels.
Sound radiation patterns from any individual speaker unit are wide and approach omnidirectional at lower frequencies where the wavelengths are large compared to the size of the speaker diaphragm. In contrast, at higher frequencies, where wavelengths are small compared to the speaker diaphragm, the sound radiation patterns become narrow and start to beam.
To create sound, a speaker diaphragm vibrates in response to an inputted electrical signal. When moving forward the speaker diaphragm causes a compression of air to occur to the front and a simultaneous rarefaction to occur to the rear of the diaphragm. The waves of alternating compression and rarefaction are 180 degrees out of phase with each other and, if allowed to meet, would cancel each other causing a void or null area. When the air is compressed on one side of the diaphragm, the air from that side rushes to the decompressed side in an attempt to equalize pressures, thus cancelling the sound waves emanating from the opposite side.
At higher frequencies the same speaker unit is very directional and radiates a narrow beam of sound to the front and the rear at equal intensity. Since the speaker diaphragm is moving at a very high rate, the wavelengths are short in relation to the diaphragm, and the air does not have time to travel from one side to the other; therefore, little effect from cancellation is noted.
As the frequency decreases, the radiation pattern broadens, and the out-of-phase sound waves meet at the edges of the speaker unit and begin to cancel. At even lower frequencies, the diaphragm is moving at a slower rate, and the wavelengths are large in relation to the diaphragm. The air then has time to travel around the edges of the speaker unit to the other side and causes almost total cancellation of the front and rear sound waves.
By extending the edges of the speaker or mounting the speaker in a baffle such that the front and rear remain open, the sound path length from front to rear is made longer, thereby lowering the frequency at which the cancellation effects occur. This configuration is known as a dipole which, due to the sound cancellation to the sides of the baffle and the continued acoustic radiation to the front and rear of the baffles, results in a cosine or figure-8 dispersion pattern. As the audio frequency varies from high to low, the front radiation pattern changes from narrow, to wide, and back to narrow, prior to cancellation. This provides a limited frequency range, or bandwidth, at which the dispersion pattern remains somewhat more constant. The shape of the front dispersion pattern, however, is affected not only by the radiating frequency, the size of the baffle, and the size of the speaker diaphragm, but also by the proximity of obstacles and room boundaries such as a wall located to the rear of the dipole speaker.
On the other hand, by enclosing the speaker unit completely so that only the front is exposed, no rear sound wave radiation is allowed to meet and cancel the front sound wave radiation. In this configuration, however, the frontal dispersion pattern again varies extensively in relationship with the frequency and the size of the speaker diaphragm. Since the enclosure behaves as a very small room affixed to the rear of the speaker unit, internal reflections are generated that cause resonances and exert pressures upon the rear of the speaker diaphragm which, in turn, produce variations in the output sound intensity. Further, the speaker diaphragm movement is restricted by the air trapped within the enclosure. This restriction impacts upon the transient response of the speaker system by limiting the ability of the speaker diaphragm to move quickly in response to an electrical pulse. Additionally, a new form of interference called diffraction results from a box-like enclosure with abrupt edges. Diffraction causes a modulation at the edges which redistributes the sound energy outwardly and back upon the original sound waves.
In the prior art, to provide a more stable radiation or dispersion pattern, it is conventional in speaker systems to divide the audio frequency range between multiple speaker units mounted in a common enclosure. A large bass unit, or "woofer", is used for reproducing the lower frequencies. A smaller speaker unit is used for the mid-range frequencies. A still smaller treble unit, or "tweeter", is used for the higher frequencies. However, multiple unit speaker systems are employed for more than just dispersion pattern considerations such as power handling capabilities, frequency response characteristics, and distortion values.
At the higher audio frequency ranges directivity is not too difficult to achieve by virtue of the diaphragm size and possible horn-shaped speaker units which project the sound forward. In the upper middle frequencies and down to the bass frequencies the dispersion pattern can very greatly within a speaker system depending upon the number of speaker units, their size and shape, and the frequency range assigned to each speaker unit. Most techniques used in the prior art for attaining directivity at these frequencies have involved arrays of speaker units, single speaker units with very large radiating diaphragms, or extremely large horn units.
In order to reduce or control reflections and reverberation effects the prior art has employed special construction procedures for the listening areas. These construction techniques involve the use of special room dimension ratios, non-parallel walls and ceilings, and acoustically absorptive materials on reflective surfaces. Other prior art techniques incorporate electronic devices, such as graphic and parametric equalizers, to compensate for frequency intensity variations within a very limited portion of the listening area.
One approach to solving the problem of reflection and reverberation effects is disclosed by U.S. Pat. No. 3,722,616, to Beavers. In that patent a bass loudspeaker is mounted in an enclosure having a front port through which the frontal sound waves emanate and open side or rear ports through which the rear sound waves radiated by the speaker cone exit. Beavers' loudspeaker system is designed to provide cancellation of the sound radiated to the rear of the loudspeaker such that most of the acoustic radiation is in the forward direction. Beavers additionally requires an enclosure volume with acoustical compliance in conjunction with resistive material covering the ports to provide a time delay. It should be stressed that any enclosure having both volume and ports will have a point of resonance described by the Helmholtz resonator effects. This technique is widely used in bass-reflex speaker systems because at and near resonance the ports generate sound radiation which is approximately in-phase with the front radiation, hence, adds to the total bass output. Whereas above resonance, the speaker system behaves as a sealed enclosure with little or no output from the ports. These actions are counterproductive to solving the problem of reflection and reverberation effects.